1. Field of the Invention
This invention relates generally to computer virtualization and, in particular, to a method and system for on-line replacement and changing of virtualization software
2. Description of the Related Art
The advantages of virtual machine technology have become widely recognized. Among these advantages is the ability to run multiple virtual machines on a single host platform. This makes better use of the capacity of the hardware, while still ensuring that each user enjoys the features of a “complete” computer. Depending on how it is implemented, virtualization also provides greater security, since the virtualization can isolate potentially unstable or unsafe software so that it cannot adversely affect the hardware state or system files required for running the physical (as opposed to virtual) hardware.
As is well known in the field of computer science, a virtual machine (VM) is a software abstraction—a “virtualization”—of an actual physical computer system. FIG. 1 shows one possible arrangement of a computer system 700 that implements virtualization. A virtual machine (VM) 200, which in this system is a “guest,” is installed on a “host platform,” or simply “host,” which will include a system hardware 100, that is, a hardware platform, and one or more layers or co-resident components comprising system-level software, such as an operating system (OS) or similar software layer responsible for coordinating and mediating access to the hardware resources.
As software, the code defining the VM 200 will ultimately execute on the actual system hardware 100. As in almost all computers, this hardware 100 will include one or more CPUs 110, some form of memory 130 (volatile or non-volatile), and one or more devices 170 (including storage devices such as a disk), which may be integral or separate and removable.
In many existing virtualized systems, the hardware processor(s) 110 are the same as in a non-virtualized computer with the same platform, for example, the Intel x-86 platform. Because of the advantages of virtualization, however, some hardware processors have also been developed to include specific hardware support for virtualization.
Each VM 200 will typically mimic the general structure of a physical computer and as such will usually have both virtual system hardware 201 and guest system software 202. The virtual system hardware 201 typically includes at least one virtual CPU 210, virtual memory (VMEM) 230, and one or more virtual devices (VDEVICE) 270 (including at least one virtual disk or similar virtualized mass storage device). All of the virtual hardware components of the VM 200 may be implemented in software to emulate corresponding physical components. The guest system software 202 includes a guest operating system (OS) 220 and drivers 224 as needed, for example, for the various virtual devices 270.
To permit computer systems to scale to larger numbers of concurrent threads, systems with multiple CPUs—physical or logical, or a combination—have been developed. One example is a symmetric multi-processor (SMP) system, which is available as an extension of the PC platform and from other vendors. Essentially, an SMP system is a hardware platform that connects multiple processors to a shared main memory and shared I/O devices. Yet another configuration is found in a so-called “multicore” architecture, in which more than one physical CPU is fabricated on a single chip, with its own set of functional units (such as a floating-point unit and an arithmetic/logic unit ALU), and can execute threads independently; multi-core processors typically share only very limited resources, for example, some cache. Still another technique that provides for simultaneous execution of multiple threads is referred to as “simultaneous multi-threading,” in which more than one logical CPU (hardware thread) operates simultaneously on a single chip, but in which the logical CPUs flexibly share not only one or more caches, but also some functional unit(s) and sometimes also the translation lookaside buffer (TLB).
Similarly, a single VM may (but need not) be configured with more than one virtualized physical and/or logical processor. By way of example, FIG. 1 illustrates multiple virtual processors 210, 211, . . . , 21m (VCPU0, VCPU1, . . . , VCPUm) within the VM 200. Each virtualized processor in a VM may also be multi-core, or multi-threaded, or both, depending on the virtualization. This invention may be used to advantage regardless of the number of processors the VMs are configured to have.
If the VM 200 is properly designed, applications (APPS) 260 running on the VM will function as they would if run on a “real” computer, even though the applications are running at least partially indirectly, that is via the guest OS 220 and virtual processor(s). Executable files will be accessed by the guest OS 220 from the virtual disk or virtual memory 230, which will be portions of the actual physical disk or memory 130 allocated to that VM 200. Once an application 260 is installed within the VM 200, the guest OS 220 retrieves files from the virtual disk just as if the files had been pre-stored as the result of a conventional installation of the application. The design and operation of virtual machines in general are known in the field of computer science.
Some interface is generally required between the guest software within a VM 200 and the various hardware components and devices in the underlying hardware platform. This interface—referred to in this text as “virtualization software”—may include one or more software components and/or layers, possibly including one or more of the software components known in the field of virtual machine technology as “virtual machine monitors” (VMMs), “hypervisors,” or virtualization “kernels.” Because virtualization terminology has evolved over time and has not yet become fully standardized, these terms do not always provide clear distinctions between the software layers and components to which they refer. For example, “hypervisor” is often used to describe both a VMM and a kernel together, either as separate but cooperating components or with one or more VMMs incorporated wholly or partially into the kernel itself; however, “hypervisor” is sometimes used instead to mean some variant of a VMM alone, which interfaces with some other software layer(s) or component(s) to support the virtualization. Moreover, in some systems, some virtualization code is included in at least one “superior” VM to facilitate the operations of other VMs. Furthermore, specific software support for VMs is sometimes included in the host OS itself.
Unless otherwise indicated, the invention described below may be used in virtualized computer systems having any type or configuration of virtualization software. Moreover, the invention is described and illustrated below primarily as including one or more virtual machine monitors that appear as separate entities from other components of the virtualization software. This is only for the sake of simplicity and clarity and by way of illustration—as mentioned above, the distinctions are not always so clear-cut. Again, unless otherwise indicated or apparent from the description, it is to be assumed that the invention can be implemented anywhere within the overall structure of the virtualization software.
By way of illustration and example only, the figures show each VM running on a corresponding virtual machine monitor. The description's reference to VMMs is also merely by way of common example. A VMM is usually a software component that virtualizes at least one hardware resource of some physical platform, so as to export a hardware interface to the VM corresponding to the hardware the VM “thinks” it is running on. As FIG. 1 illustrates, a virtualized computer system may (and usually will) have more than one VM, each of which may be running on its own VMM.
The various virtualized hardware components in the VM 200, such as the virtual CPU(s) 210, etc., the virtual memory 230, and the virtual device(s) 270, are shown as being part of the VM 200 for the sake of conceptual simplicity. In actuality, these “components” are often implemented as software emulations included in the VMM 300. One advantage of such an arrangement is that the virtualization software may (but need not) be set up to expose “generic” devices, which facilitate, for example, migration of VM from one hardware platform to another.
Different systems may implement virtualization to different degrees—“virtualization” generally relates to a spectrum of definitions rather than to a bright line, and often reflects a design choice in respect to a trade-off between speed and efficiency on the one hand and isolation and universality on the other hand. For example, “full virtualization” is sometimes used to denote a system in which no software components of any form are included in the guest other than those that would be found in a non-virtualized computer; thus, the guest OS could be an off-the-shelf, commercially available OS with no components included specifically to support use in a virtualized environment.
In contrast, another concept, which has yet to achieve a universally accepted definition, is that of “para-virtualization.” As the name implies, a “para-virtualized” system is not “fully” virtualized, but rather the guest is configured in some way to provide certain features that facilitate virtualization. For example, the guest in some para-virtualized systems is designed to avoid hard-to-virtualize operations and configurations, such as by avoiding certain privileged instructions, certain memory address ranges, etc. As another example, many para-virtualized systems include an interface within the guest that enables explicit calls to other components of the virtualization software. For some, para-virtualization implies that the guest OS (in particular, its kernel) is specifically designed to support such an interface. According to this view, having, for example, an off-the-shelf version of Microsoft Windows XP as the guest OS would not be consistent with the notion of para-virtualization. Others define para-virtualization more broadly to include any guest OS with any code that is specifically intended to provide information directly to the other virtualization software. According to this view, loading a module such as a driver designed to communicate with other virtualization components renders the system para-virtualized, even if the guest OS as such is an off-the-shelf, commercially available OS not specifically designed to support a virtualized computer system.
Unless otherwise indicated or apparent, this invention is not restricted to use in systems with any particular “degree” of virtualization and is not to be limited to any particular notion of full or partial (“para-”) virtualization.
In addition to the distinction between full and partial (para-) virtualization, two arrangements of intermediate system-level software layer(s) are in general use as, or as part of, the virtualization software—a “hosted” configuration (illustrated in FIG. 2) and a non-hosted configuration (illustrated in FIG. 1). In a hosted virtualized computer system, an existing, general-purpose operating system forms a “host” OS 420 that is used to perform certain input/output (I/O) operations, alongside and sometimes at the request and direction of a virtualization software component such as the VMM 300. The host OS 420 usually includes drivers 424 and supports applications 460 of its own, and the VMM 300 (or similar component) are both able to directly access at least some of the same hardware resources, with conflicts being avoided by a context-switching mechanism. The Workstation product of VMware, Inc., of Palo Alto, Calif., is an example of a hosted, virtualized computer system, which is also explained in U.S. Pat. No. 6,496,847 (Bugnion, et al., “System and Method for Virtualizing Computer Systems,” 17 Dec. 2002).
In addition to device emulators 370, FIG. 2 also illustrates some of the other components that are also often included in the VMM of a hosted virtualization system; many of these components are found in the VMM of a non-hosted system as well. For example, exception handlers 330 may be included to help context-switching (see again U.S. Pat. No. 6,496,847), and a direct execution engine 310 and a binary translator 320, often with an associated translation cache 325, may be included to provide execution speed while still preventing the VM 200 from directly executing certain privileged instructions in systems that do not otherwise provide such protection (see U.S. Pat. No. 6,397,242, Devine, et al., “Virtualization System Including a Virtual Machine Monitor for a Computer with a Segmented Architecture,” 28 May 2002). In many cases, it may be beneficial to deploy VMMs on top of a software layer—a kernel 600—constructed specifically to provide efficient support for the VMs. This configuration is frequently referred to as being “non-hosted.” Compared with a system in which VMMs (or other software components or layers with similar functionality) run directly on the hardware platform (such as shown in FIG. 2), use of a kernel 600 offers greater modularity and facilitates provision of services (for example, resource management) that extend across multiple virtual machines. Compared with a hosted deployment, a kernel 600 may offer greater performance because it can be codeveloped with the VMM and be optimized for the characteristics of a workload consisting primarily of VMs/VMMs. The kernel 600 also handles any other applications running on it that can be separately scheduled, as well as any temporary “console” operating system (COS) 420 that, in some systems, is included for such operations as boot the system as a whole or enabling certain user interactions with the kernel. The console OS 420 in FIG. 1 may be of the same type as the host OS 420 in FIG. 2, which is why they are identically numbered—the main difference is the role they play (or are allowed to play, if any) once the virtualized computer system is loaded and running. One example of a non-hosted, virtualized computer system is described in U.S. Pat. No. 6,961,941 (Nelson, et al., “Computer Configuration for Resource Management in Systems Including a Virtual Machine,” 1 Nov. 2005.)
The present invention as described herein may be used to advantage in both a hosted and a non-hosted virtualized computer system, regardless of the degree of virtualization, in which the virtual machine(s) have any number of physical and/or logical virtualized processors. The present invention may also be implemented directly in a computer's primary OS, both where the OS is designed to support virtual machines and where it is not. Moreover, the invention may even be implemented wholly or partially in hardware, for example in processor architectures intended to provide hardware support for virtual machines.
In the description of the preferred embodiment of the invention below, the term “hypervisor” is used to refer collectively to any software layer(s) and component(s) that are or perform the functions of a virtual machine monitor and kernel, including the case in which the VMM(s) and kernel are implemented as a single body of code.
Certain situations arise in which the hypervisor needs to be replaced, updated or changed to another version. For example, a new version of the hypervisor may have to be installed on the system hardware 100 to upgrade the hypervisor. Conventional virtualized systems required the hypervisor and all VMs and applications running on the hypervisor to be shut down to install a new version of the hypervisor, because the conventional virtualized systems were not able to support two or more instances of the hypervisor sharing the system hardware 100. Shutting down the hypervisor has the disadvantage of disrupting the operation of the VMs and the applications running on the VMs, causing downtime for system maintenance.
There has been some research in the areas of running multiple hypervisors on system hardware. The Cellular Disco project is an example of such research, and is described in Kinshuk Govil et al., “Cellular Disco: resource management using virtual clusters on shared-memory multiprocessors,” 17th ACM Symposium on Operating Systems Principles, Published as Operating Systems Review 34(5): pp. 154-0169, December 1999. However, this Cellular Disco paper merely describes partitioning hardware resources into separate pieces and running multiple VMs on different, partitioned pieces of the hardware resources in general, but does not disclose the concept of transferring control of hardware resources from one hypervisor to another hypervisor so that another hypervisor can be installed without disrupting the operation of the VMs. The Cellular Disco paper also describes allowing different instances of the hypervisor to temporarily loan and borrow hardware resources, but does not disclose the concept of allowing for permanent transfer of the control and ownership of the hardware sources from one hypervisor to another hypervisor. There has been other research in related areas, such as cluster-based hypervisor replacement which uses new hardware resources and changes from one hypervisor running on one set of hardware resources to another hypervisor running on another separate set of hardware resources, but such cluster-based hypervisor replacement fails to solve the problem of changing to another hypervisor to run on the same set of hardware resources.
Therefore, there is a need for a technique for changing from one hypervisor to another hypervisor to run on the same hardware resources in a virtualized system without disrupting the operation of the virtual machines.